Couplings between pipes must frequently be made in environments where it is difficult to manipulate conventional tools, such as wrenches, used to engage the couplings. In such environments, a great advantage is provided by self-swaging couplings that can form swaged couplings without the use of conventional tools. By using shape memory alloys ("SMA") in couplings, pipes can be swaged together using heat to activate the couplings rather than conventional tools.
Shape memory alloys possess the useful characteristic of being capable of changing physical dimensions upon heating above a transition temperature, A.sub.f, between a soft martensite phase and a hard austenite phase of the alloys. An SMA object can be processed while in a high temperature austenitic phase to take on a first shape. After cooling the SMA object below M.sub.f in the martensite phase without change of physical dimensions to memorize the first shape, the SMA object can be mechanically deformed into a second shape. The SMA object will remain in this second shape until heated to a temperature above A.sub.f, at which time the SMA object will transform to austenite and revert to its memorized first shape. An SMA member can exert large forces on adjacent members during the heat activated transition from the second shape to the first shape.
Coupling members incorporating SMA elements are well known in the art. In many conventional SMA couplings, a cylindrical element of SMA material either contracts or expands at the transformation temperature, A.sub.f, to apply a radial swaging force. Examples of these conventional SMA couplings can be found in U.S. Pat. Nos. 5,508,936; 4,951,978, 4,489,964; 4,469,357; 4,455,041; 4,424,991; 4,379,575; 4,314,718; 4,310,183; 4,283,079; 4,281,841; 4,226,448; 4,198,081; 4,149,911; 4,135,743; 4,035,007; 3,913,444 and 3,872,573.
U.S. Pat. No. 5,508,936 ("Kapgan") discloses a tubular coupling having an SMA collar with two internal teeth, one of which is swaged onto a tube by heating the collar, while the other is mechanically deformed to cause it to bite into the tube by drawing a tapered sleeve axially over the tooth. Screw threads provide the means for axial movement.
In addition to ring-shaped SMA elements which decrease in diameter upon thermal activation to effect self-swage couplings, a length reduction in SMA rods upon thermal activation has also been used to effect a radial compression. For example, U.S. Pat. No. 4,489,964 ("Kipp") discloses a connector for joining pipe ends in which the linear contraction of SMA rods draws together wedges at the ends of the rods, and these wedges then radially compress a collar element. Devices utilizing transformation of SMA material to effect a linear force used to produce a rotational motion are disclosed in U.S. Pat. Nos. 4,563,876 ("Banks"); 4,544,988 ("Hochstein") and 4,509,517 ("Zibelin"). U.S. Pat. No. 3,801,964 ("Dorrell") discloses a ring element of SMA to lock the joint of an electrical connector. U.S. Pat. No. 4,561,683 ("Lumsden") provides a pipe coupling of the screw-thread type.
Conventional SMA couplings suffer a number of limitations resulting from the presence of SMA material in the swaged couplings. For instance, a service and storage temperature limitation arises with conventional SMA couplings because cooling a swaged SMA coupling to a temperature below M.sub.f causes the SMA material to revert to a soft martensite phase which can result in weakening of the swaged coupling. Mother disadvantage is the weight and cost limitations due to the relatively high weight and density of SMA material as compared to other materials used in mechanical couplings, and the requirement that conventional SMA couplings incorporate a large amount of expensive SMA material to generate a swaging force. A further disadvantage is that SMA materials can be difficult to machine into the shapes required by some conventional SMA coupling designs.
There is a need in the art for a coupling that overcomes the service and storage temperature restrictions, the relatively high weight, the expense and the manufacturing difficulties associated with conventional SMA couplings, while preserving the major advantages of these couplings.